The present invention relates to a method for the formation of a patterned resist layer or, more particularly, to an efficient method for the formation of a very finely patterned resist layer of high fidelity to the photomask pattern having an excellently orthogonal cross sectional profile with high photosensitivity on a substrate surface by using a positive-working chemical-amplification photoresist composition.
As a trend in the manufacturing technology of semiconductor devices and liquid crystal display panels in recent years, extensive investigations are now under way to establish a photolithographic patterning process of a resist layer having a pattern resolution of as fine as 0.25 .mu.m or even finer by the use of a positive-working chemical-amplification photoresist composition. Turning now to the problem of the light source for the pattern-wise exposure of the photoresist layer to comply with the requirement in the manufacture of semiconductor devices to accomplish finer and finer patterning, a photolithographic patterning technology for obtaining a patterned resist layer of 0.15 to 0.22 .mu.m fineness by using a KrF excimer laser beam is the current target of the development works.
With an object to comply with the above mentioned requirements, a proposal is made in Japanese Patent Kokai 7-209868 for a positive-working chemical-amplification photoresist composition containing, as the film-forming resinous ingredient, a copolymeric resin consisting of hydroxyl group-containing styrene units, styrene units and tert-butyl (meth)acrylate units in a molar ratio of 40:20:40 or 33:17:50. The there proposed photoresist composition using a copolymeric resin with a relatively small amount of the hydroxyl group-containing styrene units or a relatively large amount of the tert-butyl (meth)acrylate units is not quite satisfactory when an extremely fine patterned resist layer of 0.15 to 0.22 .mu.m fineness is formed therewith because the cross sectional profile of the patterned resist layer is not fully orthogonal as desired.
In the photolithographic patterning technology, studies have been made on the temperatures in the drying heat treatment or, namely, pre-exposure baking treatment of the coating layer of the photoresist composition as applied to the substrate surface and the post-exposure baking treatment after pattern-wise exposure of the resist layer to light. It is known that the pre-exposure baking treatment is undertaken preferably at a temperature higher than the temperature in the post-exposure baking treatment (see, for example, Journal of Vacuum Science and Technology, 278, March/April, 1991). This teaching, however, is not applicable to various types of photoresist compositions because the studies reported there were conducted by using a positive-working photoresist composition comprising a novolak resin and a polyhydroxystyrene resin of which a part of the hydroxy groups were substituted by tetrahydropyranyl groups, which necessarily had low transmissivity to ultraviolet light giving only a limited critical pattern resolution.
Japanese Patent No. 2688188 teaches a photolithographic patterning method in which the pre-exposure baking treatment is conducted at a temperature not lower than the glass transition temperature of the resinous ingredient by more than about 20.degree. C. but not exceeding the scission temperature of the acid-labile groups and the post-exposure baking treatment is conducted at a temperature of about 110.degree. C. or higher. In this method, however, the temperature of the post-exposure baking treatment is not necessarily lower than the temperature of the pre-exposure baking treatment so as not to attain a pattern resolution of 0.22 .mu.m or finer.
In recent years, on the other hand, onium salts or, in particular, those containing sulfonate ions as the anion are highlighted as an acid-generating agent used in positive-working chemical-amplification photoresist compositions. Examples of such an onium salt as the acid-generating agent in a positive-working chemical-amplification photoresist composition heretofore proposed include onium salts containing a fluoroalkyl sulfonate ion and having at least one acid-instabilizable group on the phenyl group in the molecule such as 2,2,2-trifluoroethane sulfonic acid (4-tert-butoxyphenyl) diphenyl sulfonium as disclosed in Japanese Patent Kokai 10-7650.
The onium salts having an acid-instabilizable group in the molecule, however, are defective due to their low solubility in the organic solvent of the photoresist solutions to limit the amount thereof in the photoresist solution resulting in a decreased photosensitivity and low storage stability of the photoresist solution. In addition, the acid-instabilizable group is liable to be decomposed when the pre-exposure baking treatment or, namely, heat-drying of the photoresist layer formed by coating is performed at 120.degree. C. or higher eventually to lose the activity as an acid-generating agent.